Although conventional lasers emitting coherent radiation at a single wavelength have become indispensable research tools, for many applications there is still a need for a laser emitting simultaneously at a plurality of predetermined wavelengths or with a predetermined spectral composition. The realization of a polychromatic or white light laser provides novel approaches in numerous fields such as medicine, spectroscopy, holography, photo-chemistry, isotope separation, spectrum analysis, optical measurement, and/or ultra-short light pulse generation. In medicine, for example, white light lasers may be used to irradiate human organs with radiation whose spectrum is optimal with respect to the absorption characteristics of the organ under treatment. Accordingly, the development of polychromatic or white light lasers is of a particular interest to the scientific community. See also, for example, "White Light Laser," Applied Optics, Vol. 9, No. 5, p. 1209 (1970).
With respect to polychromatic or white light lasers, selected documents as discussed below are of interest.
U.S. Pat. No. 3,928,817 discloses a laser resonator for achieving forced oscillation at multiple wavelengths. Multiple-selected line operation is achieved with diffraction gratings and mirrors forming an independent three-element cavity for each desired wavelength. Oriented at the appropriate angle, the diffraction gratings allow regenerative amplification for the vibrational levels of the active medium disposed within the cavity.
U.S. Pat. No. 4,298,486 discloses laser resonator cavities comprising at least one active medium disposed in an optical cavity. A pair of similar intracavity spectral dispersion means allow radiation of different wavelengths to be regeneratively amplified in co-parallel regions of the active medium. Moreover, apertures within the cavity are positioned to transmit and block selected radiation wavelengths to produce forced oscillation at more than one wavelength.
U.S. Pat. No. 4,759,026 discloses a dye laser capable of lasing simultaneously at a plurality of wavelengths. A plurality of transparent containers transversely staggered are positioned between a partially-reflecting mirror and an adjustable retro-reflecting diffraction grating. The containers enclosing three separate dye solutions are pumped to generate spontaneous emission from each of the dye solutions. With the diffraction grating operating in three different diffraction orders, three parallel, but staggered cavities, are formed to effect lasing at three different wavelengths.
PCT application PCT/SU 89/00163 and Russian patent SU 1718313 disclose a white light laser comprising an active medium disposed within an optical cavity. The optical cavity comprises a diffraction grating operating in an auto-collimation mode, a pair of achromatic lenses positioned on opposite sides of the active medium, and a mirror. The diffraction grating and mirror form the ends of the optical cavity. Through the use of optical waveguides, the active medium is pumped along discrete portions. Spontaneous radiation emitted by the active medium propagates through one of the achromatic lenses and is then incident on the diffraction grating as collimated beams of varying angles of incidence. In accordance with the diffraction grating equation, for each collimated beam, only one wavelength component is diffracted back along the propagation direction onto the corresponding pumped portion of the active medium. Subsequently, the back reflected radiation is amplified by the active medium and then focused by the second achromatic lens onto the mirror. With the mirror reflecting the amplified radiation back through the active medium, simultaneous lasing is achieved for different discrete wavelengths.
Although the above polychromatic or white light lasers perform acceptably, the optical cavity structures are somewhat complex and large. Moreover, the overall performance, such as the spectral linewidth, intracavity loss, and optical efficiency, is limited and critically dependent on the complexity of the cavity structure.
It is therefore desirable to develop polychromatic or white light lasers not only having a simple cavity structure, but also having an enhanced and controllable spectrum.